The optoelectronic and structural properties of hydrogenated amorphous silicon-carbon alloys (a-SiC:H) are studied over the entire compositional range of carbon content. The films are prepared using low-power electron-cyclotron resonance (ECR) plasma-enhanced chemical vapor deposition. The carbon content was varied by using different methane (or ethylene-)-to-silane gas phase ratios and by introducing the methane (or ethylene) either remotely into the plasma stream or directly through the ECR source, together with the excitation gas (hydrogen). Regardless of the deposition conditions and source gases used, the optical, structural and transport properties of the a-SiC:H alloys followed simple universal dependencies related to changes in the density of states associated with their structural disorder. The deep defect density from photothermal deflection spectroscopy, the ESR spin density, the steady state and the transient photoluminescence, the dark and photoconductivity, the temperature of the hydrogen evolution peaks and the bonding from infrared spectroscopy are correlated to the Urbach tail energy, the B factor of the Tauc plot and E04 (defined as the energy at which the absorption coefficient is equal to 104 cm−1). Silicon-rich and carbon-rich regions with very different properties, corresponding approximately to carbon fractions below and above 0.5, respectively, can be distinguished. The properties of the ECR a-SiC:H alloys are compared with those of alloys deposited by rf glow discharge. © 1999 American Institute of Physics.